1. Field of the Invention
The present invention concerns a coating machine provided with a coating material discharging chamber for previously filling a coating material and discharging the same under pressure to an atomizing mechanism during coating and, more specifically, it relates to an electrostatic coating machine for electrostatically atomizing a conductive coating material such as an aqueous paint.
2. Statement of Related Art
In the coating of car bodies, coating materials using organic solvents are predominant, but it has been demanded to decrease volatile organic solvents that are evolved a great amount in the coating process with a view of environment protection and prevention of public pollution and, as a outer measure, coating with aqueous coating materials has attracted attention.
For using an aqueous coating with no loss, it is preferred to apply coating by an electrostatic coating apparatus of high coating efficiency. However, since the aqueous coating material has low electric resistance tending to electrically conduct the rotary atomizing head and the ground of the electrostatic coating machine by way of a coating material flowing through a coating material supply system, insulation has to be applied over the entire coating material supply system so as to prevent leakage of high voltage of −60 to 90 kV applied to the rotary atomizing head.
Accordingly, a coating material is filled in a coating material tank formed in a coating machine or a coating material is filled in a cartridge mounted detachably to a coating machine and a coating martial is discharged under pressure from the coating material tank or the cartridge for coating thereby electrically shielding the coating material supply system in order not to leak a high voltage even when it is applied to the coating machine (for example, refer to Japanese Unexamined Patent Publication No. 2000-317354).
In the electrostatic coating machine of the type described above, a bottom plate as a piston is slidably located along the inner peripheral surface of a coating material tank or a cartridge as a cylinder and the bottom plate is pushed by other actuator or under a reduced pressure to press-discharge the coating material.
However, since the bottom plate and the inner peripheral surface have to be sealed reliably, friction increases by so much to require a large driving force. Since the seal is worn by friction on every reciprocation of the bottom plate, an operating fluid may possibly enter to give an undesired effect on the quality of the coating in a case of liquid pressure driving.
Further, since plural O-rings are arranged in parallel to the outer peripheral surface of the bottom plate as a piston in the usual seal, the coating material intrudes between each of the O-rings and this imposes a trouble of decomposing and detaching the bottom plate and clean the same upon cleaning after completion of every day's job.